The formation and maintenance of the neuromuscular synapse requires the coordinated expression and assembly of a large set of synaptic macromolecules. Over the past 3 years we have begun to study the role of a number of these synaptic proteins by creating knock-out mutations in their respective genes in mice. We have the following gene knock- out mutations in various stages of completion: S-laminin, a component of the synaptic based lamina, made by muscle, and predicted to be an adhesive molecule that may signal the nerve terminal to differential; 43k/rapsyn, a cytoplasmic extrinsic membrane protein that is hypothesized to be a structural component of the acetylcholine receptor (AChR) clustering mechanism; Agrin, a nerve derived component of the extracellular matrix that is hypothesized to be the signal that induces AChR clustering; AChR-epsilon subunit, the adult specific subunit of the AChR; DRP/utrophin, a novel homologue of dystrophin that is highly enriched at the synapse, and hypothesized to play a role in the regulation of AChR clustering. The synapses of these mutant animals will be analyzed throughout development by electrophysiological, immunocytochemical, and biochemical animals.